The present invention relates to a atmosphere measuring device and more particularly to a self-compensating type atmosphere measuring device which is capable of measuring the density of gas according to a differential resistance of a resistor heated with low and high electric powers in an ambient atmosphere to be measured, and which is suited to use in various measuring devices such as for example hygrometers, gas chromatographs, vacuum gauges, dew-point hygrometers, hot-wire anemometers and so on.
It is well known that the density of a specified gas contained in a mixed atmosphere can be thermally detected on the basis of a difference of thermal conductivities of the gas, which varies depending upon its molecular weight. Among atmosphere measuring devices working on this measuring principle, hygrometers find particularly extensive application, for example, as humidity controllers for quality control in the production process of electronic parts (e.g., semiconductors), optical precision devices, fibers, foods and other goods, and as sensing terminals for environmental control in hospitals, buildings and like installations.
In view of these circumferences, a hygrometer will be described below as a preferred embodiment of the present invention, but the present invention is not limited to said application and widely relates to general atmosphere measuring devices for measuring the density of a mixed gas and, more practically, to above-mentioned various measuring devices.
Hygrometers are roughly classified into two groups according to measuring principles--one is electrical variation detecting type and the other is mechanical variation detecting type. Both types pose problems on their reliability and duration of life, and many of them have poor response characteristics.
The electric hygrometers using the thermal conductivity of gas are known to be of high response and high reliability. The heat that flows in unit time in the normal direction passing an upper and a lower planes of a specified cross section of an isotropic body is proportional to the temperature gradient and the cross-sectional area. This proportional constant is thermal conductivity. The thermal conductivity of a gas is a function of isopiestic specific heat that is further a function of the gas molecular weight. Therefore, the thermal conductivity of only air differs from the thermal conductivity of air containing therein moisture and gas components having different molecular weights. There is such a hygrometer which, utilizing a difference of thermal conductivities of gases, determines the humidity of an ambient atmosphere by measuring a change in resistance of a sensing element (resistor), which is caused by a heat radiation from the heated resistor into the atmosphere.
In a practical hygrometer, a temperature-compensating element which has a resistor mounted in a closed compartment filled with dry air at a constant pressure and a sensing element which has a resistor exposed to an atmosphere whose humidity is to be measured are disposed close to each other. Both resistors are heated with a constant electric power and change their resistance values. A change of resistance by the effect of the humidity (moisture content of the atmosphere) is determined by reducing, i.e.--subtracting, the resistance value of the temperature compensating element from the resistance value of the sensing element.
However, the hygrometers which use a combination of a two elements (a temperature-compensating element and a humidity-sensing element) have such a common problem that the temperature-compensating element, if being filled with dry air at a pressure other than the specified constant pressure, can not serve as a reference. If the dry air has a different pressure, its thermal conductivity changes and the temperature sensing element has a different temperature-sensing characteristic.
Furthermore, the specified pressure of the dry air in the temperature compensating element serves as a reference for compensation of measurement when sensing the humidity of an atmosphere, but thermal conductivity of the dry air may vary with a change of the ambient pressure (of an atmosphere containing a moisture), reducing its effectiveness of the reference. Accordingly, the humidity of the dry air and the dry air filling conditions shall be very strictly controlled because any variation of these conditions leads to variations of the characteristics of the products. Furthermore, a combination of a sensing element with a temperature-compensating element requires complicated matching operations. All the above-mentioned facts lead to decreasing of the yield of products.
The pressure of the dry air charged in an enclosure of the temperature-compensating element is constant, but the ambient atmosphere may change its pressure under different meteorological conditions (e.g., at a highland place) and therfore change its thermal conductivity. If the pressure of the dry air in the enclosure of the temperature-compensating element can vary according to the working conditions with change of the outside air pressure, the temperature compensation is possible. But, sealing caps of prior art devices are not so flexible in its material and construction as it may be deformed to change its inner pressure with change of the atmospheric pressure. Consequently, the prior art devices can not get the correct value of the humidity under the above-mentioned conditions. When the atmosphere to be detected is partially dense and the sensing element and temperature-compensating element are widely separated from each other, detection signals represent only comparison values obtained at separate (not the same) positions and they can not be accepted as the data obtained under the same conditions. Therefore, it is desired to dispose the sensing element and the temperature-compensating element at a possibly minimal distance to each other. For instance, it has been proposed to arrange these elements on one substrate, but further close arrangement of the elements has been still required.
The above-mentioned conventional atmosphere measuring devices, each using a combination of a separate sensing element and a separate temperature-sensing element, have such drawbacks that they can not be applicable for wide range of humidity measurement and can not attain a higher sensing performance if elements have not matched characteristics of resistance (volt-ampere characteristic), resistance temperature coefficient, response time (building-up time) and aging. These severe requirements on matched characteristics of paired elements lead to increasing their manufacturing cost because of correspondingly reduced mass productivity and product yield and of increased man-hours. The conventional device consumes a relatively large electric energy since its sensing element and temperature compensating element are powered from respective power sources. The present invention also relates to a flow sensor and, more particularly, to a flow sensor which is suitable for use in a thermal type flowmeter based upon that electrical resistance value of a resistor disposed in a flowing fluid and heated therein varies with change of the fluid temperature and flow rate.
The thermal flowmeters having no moving portion thanks to their compactness and high-speed response have been now used in various fields of application. Recently, the thermal flowmeters have drawn the increasing attention as a device suited for regulating air flow so as to get an optimal air-fuel ratio in internal-combustion engines. A resistor heated at a specified electric power becomes to have a temperature at which the heat generated from the heating electrical energy and the heat radiated by a fluid flow are balanced with each other. Consequently, the heat radiation is a function of the flow rate, isopiestic specific heat and density of a fluid. In any thermal flowmeter based on this principle, a resistor disposed in a gas flow is heated with a constant electric power, a measured resistance value of the resistor is reduced by a resistance value corresponding to a measured temperature of the fluid to eliminate the thermal influence of the fluid, and the mass flow rate of the fluid is determined from the corrected resistance value of the resistor.
Practically, any conventional thermal flowmeter adopts such a system in which a heater-resistor (heat-generating element) generates a heat and a heat-sensing resistor (heat receiving element) disposed at a specified distance therefrom senses the transferred heat. The conventional flowmeter needs to use at least one pair of resistor elements (heat-generating element and heat-receiving element) which must be identical in size, specific heat and heat capacity, thereby it involves the following problems to be decided.
[1] Variations in resistance of a heat-generating element and a heat-receiving element:
(1) It is required to correctly adjust electric circuits to compensate a variation of temperature (i.e., resistance value) of the heat-generating element and the variations of temperature (i.e., resistance value) of the heat receiving element respectively; PA1 (2) Regarding the temperature balance, i.e., resistance balance between the heat-generating element and the heat-receiving element, both elements are formed on a semiconductor substrate by using micro-machining technology of IC production and may be generally said to be combined with relatively high accuracy. However, the elements may be of no use if their resistance values are not sufficiently balanced. The conventional coil type thermal flowmeter may use a combination of a separate heating-generating element and a separate heat-receiving element, which are selected as well balanced in their resistance characteristics, thereby it may be manufactured with higher productivity than the thermal flowmeter having the elements integrally formed on a substrate; PA1 (3) It is difficult to arrange the heat-generating element and the heat-receiving element at the optimal positions and at an optimal distance. The elements are also limited by an attainable accuracy; PA1 (4) Since there is a distance between the heat-generating element and the heat-receiving element, it is impossible to correctly detect a difference of resistance values for a very small flow-rate of fluid; PA1 (5) Both the heat-generating element and the heat-receiving element shall have a high accuracy of temperature distribution. Any dispersion of the temperature distribution may affect a detection output signal and makes it necessary to additionally adjust each respective sensor circuit; PA1 (6) Increasing the number of the heat-generating and heat-receiving elements increases the size of the substrate and the number of its output leads. When the substrate of an increased size is disposed in a stream of fluid to be measured, it may prevent the fluid from freely flowing and can not obtain the correct measurement result. This also leads to increase of the manufacturing cost of the product. The accuracy of sensing a small flow rate is also reduced; PA1 (7) The increased number of heat generating and receiving elements increases the electric power consumption; PA1 (8) The vertical arrangement of the heat-receiving element at the upper position and the heat-generating element at the lower position in a vertically upward stream of gas may be encountered with such a problem that even with no flow of fluid to be sensed, the heat-receiving element generates an output signal, sensing an ascending current of air heated by the heat-generating element. This means that thus constructed flowmeter shall be mounted with special consideration of its mounting conditions and has a limitation of its mounting places. PA1 (1) In a flowmeter, wherein a set of three micro-bridge-like elements (resistances) disposed at top, center and last positions from the upstream side in gas passage is used to detect a temperature of the gas flow by the top element and a flow-rate of the gas by the center heat-generating element and the last heat-receiving element and to compensate the measured flow rate for variation of temperature according to the information obtained by the top element, the drawbacks (1) to (8) described above in item [1] regarding the variations of resistance values of the heat-generating element and the heat-receiving elements become more serious; PA1 (2) A flowmeter, wherein a pattern of a resistor for sensing an ambient temperature is formed on a semiconductor substrate, can not serve to correctly detect a flow rate because its resistance value slowly responses and changes with a change of temperature of the semiconductor substrate having a large heat capacity, thereby the correct temperature of the gas at the moment of its flow-rate measurement can not be detected.
[2] Regarding compensation for an ambient temperature when detecting a gas flow: